Enzymatic methylation of arsenic compounds. VI. Characterization of hamster liver arsenite and methylarsonic acid methyltransferase activities in vitro.
Enzymatic methylation of arsenic compounds. VI. Characterization of hamster liver arsenite and methylarsonic acid methyltransferase activities in vitro.
复制标题
砷化合物的酶促甲基化。
DOI：
10.1006/taap.1998.8409
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
Aposhian,HV
中科院分区：
文献类型：
--
作者：
Wildfang,E;Zakharyan,RA;Aposhian,HV
无机砷的甲基化反应被认为是无机砷生物转化和解毒的主要途径。体内的比较研究表明，动物对无机砷甲基化的能力存在差异。我们认为无机亚砷酸盐甲基化的速率可能是导致观察到的物种变异的因素之一。金黄地鼠肝的亚砷酸盐和MMA甲基转移酶分别被部分纯化了40倍和67倍。单硫醇-半胱氨酸在体外促进这些酶的活性，比类似浓度的谷胱甘肽或二硫苏糖醇更大。部分纯化的亚砷酸盐和MMA甲基转移酶的最适pH分别为7.6和8.0。这两种酶的活性都表现出经典的米氏酶动力学。金黄地鼠肝砷酸甲基化转移酶的KmandVmax分别为1.79×10~(-6)−~(-6)和0.022 pmol/mg蛋白质/60min。金黄地鼠肝甲基转移酶的KmandVmax分别为7.98×10-4M和0.007−/mg蛋白/60min。在兔子的肝脏中也观察到了类似的这些活动的动力学关系，像仓鼠一样，它比大多数其他被研究的物种分泌更多的MMA。在这两个物种中测量到的MMA甲基转移酶比亚砷酸盐甲基转移酶更高的Kmand和更低的Vmax表明，MMA的产生速度可能高于随后将其甲基化为DMA的速度，从而允许MMA积累和排泄。
Methylation of inorganic arsenic to methylarsonic acid (MMA) and dimethylarsinic acid (DMA) has been considered to be the major pathway of inorganic arsenic biotransformation and detoxification. Comparative studies,in vivo,have demonstrated variation in the abilities of animals to methylate inorganic arsenic. We propose that the rate of inorganic arsenite methylation may be one of the factors responsible for observed species variation. Arsenite and MMA methyltransferases of Golden Syrian hamster liver have been partially purified 40- and 67-fold, respectively. The monothioll-cysteine promotes greater activities,in vitro,of these enzymes than similar concentrations of either glutathione or dithiothreitol. The pH optima of the partially purified arsenite and MMA methyltransferase activities are 7.6 and 8.0, respectively. Both activities display classic Michaelis–Menten enzyme kinetics. TheKmandVmaxof hamster liver arsenite methyltransferase are 1.79 × 10−6M and 0.022 pmol/mg protein/60 min, respectively. Hamster liver MMA methyltransferase hasKmandVmaxvalues of 7.98 × 10−4M and 0.007 pmol/mg protein/60 min, respectively. A similar kinetic relationship of these activities is also observed in the liver of the rabbit, which, like the hamster, excretes higher amounts of MMA than most other species studied. The higherKmand lowerVmaxof MMA methyltransferase, compared to arsenite methyltransferase, measured in these two species suggests that MMA may be produced at a rate higher than it can be subsequently methylated to DMA, thereby allowing MMA to accumulate and be excreted.
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